Doubtful that their legal description states to the centerline of the river, or if they own property on both sides it probably has an exclusion for the river. The description of "the river" is pretty loose, however it usually means OHW or edge of vegetation along the Wynoochee. A lot of land owners will use this to their advantage. I.e. defining their property boundary as the meander margin to save on taxable land, or the opposite pushing it to the edge of the water to create enough land to subdivide it.

Most likely you pay the same in taxes for the gravel bar you were standing on as the people who kicked you off of it.